The present invention relates generally to polymeric latexes exhibiting outstanding tolerance to multivalent electrolytes. More particularly, the present invention relates to polymeric latexes with high multivalent-ion stability prepared by aqueous emulsion polymerization of a monomeric mixture in the presence of a seed polymer comprising styrene and the neutralized form of 2-acrylamido-2-methylpropanesulfonic acid. The latexes may be useful in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing substantial amounts of inorganic pigments or fillers.
Most commercial latexes are classified as anionic. This means that there is a negative charge on the latex particle. This negative charge can be produced in several ways: (1) use of anionic monomers such as carboxylic or sulfonic acids or their salts; (2) the normal incorporation of anionic initiator fragments derived from persulfates; and (3) adsorption of the anionic surfactants used to generate latex particles and stabilize their growth. Of course, like all salts there is an oppositely charged counterion that is relatively free in the water phase to keep the overall charge balanced.
The negative charge on the latex particle plays a crucial part in its keeping the latex stable. Electrostatic repulsion of the like (xe2x88x92) charges keep the particles from clumping together and forming larger clusters that eventually precipitate from the water phase.
Any variable that reduces the effective surface charge decreases the latex stability. Hence, adding simple salts to a latex can destabilize it. The cationic portion of a simple salt associates with the negative charges on the latex and reduces the overall charge at the particle surface. The effect of the cationic counterion depends upon both its concentration and its charge or valency. Thus multivalent cations are especially harmful in destabilizing anionic latex. The ionic strength is one measure of the destabilizing effect of a solution on latex. The product of the salt concentration and the square of the ionic charge determine the ionic strength; therefore, equamolar amounts of Na+, Ca++, and Al+++ have relative effects of 1, 4, and 9 respectively. By using both different multivalent salts and different concentrations, one can devise increasingly more severe latex stability tests and establish different echelons of latex stability.
The effect of temperature is also substantial. As the temperature increases, eventually the higher kinetic energy of the latex particles may allow them to overcome the electrostatic repulsion, collide and coalesce. Consequently, a combination of high electrolyte concentrations of multivalent cations and elevated temperatures constitutes an especially severe set of conditions for latex stability. Indeed, commercial latexes are considered xe2x80x9cexcellentxe2x80x9d if they can withstand the slow addition of 10 mL of 2% calcium chloride to about 50 mL of latex, even at room temperature. It is well known that as the temperature is increased then the stability of latex in the presence of salts is greatly reduced. For this reason, room temperature tests are used that call for much higher electrolyte concentrations than is actually encountered in an application so as to compensate for needing to function at higher temperatures. Also, adding a hot salt solution to hot latex is less convenient as a screening test.
In electrolyte stability testing, the amount of residue or grit that is generated when the latex is xe2x80x9cshockedxe2x80x9d by adding the salt solution is measured. Naturally, the identity of the salt and the strength of the salt solution determine the amount of residue produced. The rate of addition of the salt solution, stirring of the latex, etc. can also have an effect in discerning between borderline cases or similar stabilities. The amount of residue generated in the test is not to be confused with grit or residue that may be formed during the latex manufacturing. For this reason the latex is first filtered free of fine grit prior to testing.
It will be appreciated from the foregoing that latexes having high multivalent-ion stability may be useful in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing substantial amounts of inorganic pigments or fillers.
For example, techniques for drilling and completing wells, particularly gas and oil wells, are well established. Of chief concern here are those wells which are drilled from the surface of the earth to some subterranean formation containing a fluid mineral which it is desired to recover. After the fluid containing geologic formation is located by investigation, a bore-hole is drilled through the overlying layers of the earth""s crust to the fluid containing geologic formation in order to permit recovery of the fluid mineral contained therein. A casing is then positioned within the borehole to insure permanence of the borehole and to prevent entry into the well of a fluid from a formation other than the formation which is being tapped. This well casing is usually cemented in place by pumping a cement slurry downwardly through the well borehole, which is usually accomplished by means of conducting tubing within the well casing. The cement slurry flows out of the open lower end of the casing at the well bottom and then upwardly around the casing in the annular space between the outer wall of the casing and the wall of the well borehole.
Gas channeling is a phenomenon that occurs during the setting of the cement slurry. Once the cement slurry begins to set, the hydrostatic pressure in the cement column begins to decrease. This reduction in hydrostatic pressure allows the channeling of gas. This phenomenon occurs during setting of the cement, from the time when setting has progressed enough for the hydrostatic pressure to no longer be transmitted, or to no longer be sufficiently transmitted through the cement, but not enough for the cement at the level of the gas pocket to oppose migration of the gas into the setting cement under the pressure from the gas pocket which at this point is no longer balanced by the hydrostatic pressure.
The pressurized gas then migrates through the cement slurry in the course of its setting and/or between the cement and the drilled formations, creating a multiplicity of channels in the cement, which channels may reach up to the surface of the well. It will be appreciated that gas channeling can be exacerbated by the cement""s shrinkage and possibly by liquid losses from the cement slurry through filtration into the surrounding earth, especially in the area of porous formations, also termed xe2x80x9cfluid lossxe2x80x9d.
Gas channeling is thus a serious drawback leading to weakening of the cement and to safety problems on the surface. Various styrene-butadiene latexes have been used as an additive for oil and gas well cementing, primarily to control gas channeling. For example reference is made to U.S. Pat. Nos. 3,895,953; 3,043,790; 4,151,150 and 4,721,160, incorporated herein by reference. It will be appreciated that cements typically include calcium, aluminum, silicon, oxygen and/or sulflur and which set and harden by reaction with water. These include those cements commonly called xe2x80x9cPortland cementsxe2x80x9d, such as normal Portland or rapid-hardening or extra-rapid-hardening Portland cement, or sulfate-resisting cement and other modified Portland cements, cements commonly known as high-alumina cements, high-alumina calcium-aluminate cements. Although the latexes heretofore used have been found to function, further improved latexes are desired in systems containing alum, calcium carbonate, gypsum, zinc oxide, aluminum calcium phosphate, natural high-hardness brines, and other multivalent inorganic materials.
It is an object of the present invention to provide a polymeric latex with high multivalent-ion stability. It is another object of the present invention to provide a styrene butadiene based latex functionalized with a sulfonated acrylamide monomer that exhibits high tolerance to multivalent electrolytes, even at elevated temperatures. Another object of the present invention is to provide a latex that may be useful in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing susbstantial amounts of inorganic pigments or fillers. More particularly, it is an object of the present invention to provide a polymeric latex with high multivalent ion stability which is relatively inexpensive, and provides superior fluid loss control without adversely affecting other critical properties of the cement slurry for oil and gas well cementing. It is yet another object of the present invention to provide a polymeric latex useful as an additive for cement compositions for cementing wells. It has been discovered in accordance with the present invention, that a polymeric latex additive comprising styrene, butadiene and 2-acrylamido-2-methylpropanesulfonic acid when mixed with cement to form a slurry has the effect of limiting the porosity and blocking gas channeling. These and other objects and advantages will become more apparent from the following detailed description and examples.
Briefly, the present invention relates to a polymeric latex prepared by aqueous emulsion polymerization of a monomeric mixture comprising styrene and butadiene in the presence of a seed polymer prepared by aqueous emulsion polymerization of styrene and a salt of 2-acrylamido-2-methylpropanesulfonic acid.
Styrene butadiene based latexes functionalized with a sulfonated acrylamide monomer exhibit surprisingly high tolerance to multivalent electrolytes, even at elevated temperatures. Such latexes have potential utility in the processing and recovery of natural resources in the mining, petroleum and geothermal industries as well as in paper and textile coatings and construction mixtures employing susbstantial amounts of inorganic pigments or fillers.
The present invention is directed to polymeric latexes comprising styrene, butadiene and the neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid, also commonly known as AMPS. AMPS is a registered trademark of The Lubrizol Company. The polymeric latexes in accordance with the present invention have been found useful as an additive to cementing compositions for oil, gas, and geothermal wells. Utility is also anticipated in applications which require stability of a latex binder in systems containing alum, calcium carbonate, gypsum, zinc oxide, aluminum calcium phosphate, natural high-hardness brines, and other multivalent inorganic materials.
The polymeric latexes in accordance with the present invention are prepared via a seeded polymerization of a monomeric mixture comprising styrene and butadiene using deionized water as a continuous phase, i.e., aqueous emulsion. The ratio of styrene to butadiene in the polymeric latex is typically about 2:1, although a somewhat higher or lower ratio may be used. Preferably, the polymeric latexes include about 30 to 80 weight percent styrene and about 20 to 70 weight percent butadiene.
The seed used in the aqueous emulsion polymerization is prepared by first copolymerizing an aqueous emulsion of a mixture of about 5 to 20 weight percent of styrene monomer, preferably about 8 to 12 weight percent of styrene monomer and from about 5 to 20 weight percent of the neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid, preferably about 5 to 10 weight percent. It will be appreciated that levels of the neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid above about 10 to 20 weight percent causes a broad particle size distribution. It has been found that the salts of 2-acrylamido-2-methylpropanesulfonic acid provide superior electrolyte and high temperature resistance to the polymeric latexes in accordance with the present invention in contrast to the carboxylates, alcohols, phenolics and steric stabilizers typically used in emulsion polymerization.
The neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid may be formed by the neutralization of the acid monomer with an alkaline agent such as a source of sodium, calcium, magnesium, ammonium ions and the like to form the salt of 2-acrylamido-2-methylpropanesulfonic acid.
In an alternate embodiment, the seed may be formed by aqueous emulsion polymerization of a mixture of about 5 to 12 weight percent of styrene monomer and about 2 to 6 weight percent of butadiene monomer and from about 3 to 20 weight percent, preferably about 5 to 10 weight percent of the neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid. In yet another alternate embodiment, the seed may be formed by aqueous emulsion polymerization of a mixture of about 5 to 10 weight percent of styrene monomer and about 2 to 6 weight percent of butadiene monomer and from about 3 to 10 weight percent, preferably about 3 to 5 weight percent of the neutralized form of the monomer 2-acrylamido-2-methylpropanesulfonic acid and about 2 to 5 weight percent seed comonomer.
The seed comonomer allows the polymeric latex to reach a stability equivalent to formulations containing higher concentration levels of the neutralized form of 2-acrylamido-2-methylpropanesulfonic acid. The seed comomoners may be selected from acrylonitrile, preferably mildly hydrophobic acrylamides such as methacrylamide, N-isopropyl- and N-t-butyl acrylamide, and N-(1,1-dimethyl-3-oxobutyl)acrylamide. Also effective as a seed comonomer are di(meth)acrylates with ethylene oxide spacer units in the 5-20 range. Less preferred seed comonomers are C1-C3 (meth)acrylates. It will be appreciated that acrylamide has been found ineffective as a seed comonomer and deleterious to polymeric latex production.
The above monomers are polymerized in the presence of water, free radical initiators, anionic surfactants, and chelating agents to form the latex binder of the present invention using conventional emulsion polymerization procedures and techniques except as otherwise provided herein.
The free radical initiators utilized to polymerize the monomers of the present invention include sodium persulfate, ammonium persulfate, potassium persulfate and the like. Other free radical initiators can be utilized which decompose or become active at the polymerization temperature such as various peroxides, e.g., cumene hydroperoxide, dibenzoyl peroxide, diacetyl peroxide, dodecanoyl peroxide, di-t-butyl peroxide, dilauroyl peroxide, bis(p-methoxy benzoyl) peroxide, t-butyl peroxy pivalate, dicumyl peroxide, isopropyl percarbonate, di-sec-butyl peroxidicarbonate, various azo initiators such as azobisdimethyivaleronitrile, 2,2xe2x80x2-azobisisobutyronitrile, 2,2xe2x80x2-azobis(2-amidinopropane) dihydrochloride, 2,2xe2x80x2-azobis-2-methyl-butyronitrile, 2,2xe2x80x2-azobis(methylisobutyrate), and the like and mixtures thereof The amount of the free radical initiator is generally from about 0.1 to 2, and preferably from about 0.5 to 1.0 parts by weight per 100 parts by weight of the total amount of monomers added.
Optional chain transfer agents include mercaptans such as the alkyl and/or aryl(alkyl) mercaptans having from about 8 to about 18 carbon atoms and preferably from about 12 to about 14 carbon atoms. The tertiary alkyl mercaptans having from about 12 to about 14 carbon atoms are highly preferred. Examples of specific chain transfer agents include n-octyl mercaptan, n-dodecyl mercaptan, t-octyl mercaptan, t-dodecyl mercaptan, tridecyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan and the like, as well as mixtures thereof. The amount of the chain transfer agent utilized is from about 0.2 to 2.5, and preferably from about 0.5 to 1.5 parts by weight per 100 parts by weight of the total amount of monomers added.
The anionic surfactants include sodium dodecylsulfate, sodium dodecylbenezene sulfate, sodium dodecylnapthalene sulfate, dialkylbenzenealkyl, sulfates, sulfonates and the like, especially preferred is the dihexyl ester of sodium sulfosuccinate. The amount of anionic surfactant present is sufficient to obtain an aqueous emulsion of the monomers. Such an amount is typically from about 0.5 to 1.5 parts by weight per 100 parts by weight of the total amount of monomers added. It will be appreciated that the present invention does not require the presence of additional stabilizers, ionic surfactants, stabilizing surfactants such as ethoxylated sulfonates and the like in order to attain the high electrolyte tolerances needed.
Chelating agents may also be used during polymerization to tie up various metal impurities as well as to achieve a uniform polymerization. Examples of specific chelating agents include ethylene diamine tetra-acetic acid, nitrilotriacetic acid, citric acid, and their ammonium, potassium and sodium salts. The amounts of the chelating agents may range from about 0.01 to 0.2 parts by weight per 100 parts by weight of the total amount of monomers added.
The polymerization process is effected by the selective addition of the various reactants in multiple stages to the reaction zone of a reactor as the reaction continues. The polymerization process is generally carried out from about 120 to 200 degrees F, and preferably from about 150 to 170 degrees F.
The process includes the step of forming a first polymeric seed by charging into the reaction zone of the reactor an aqueous emulsion polymerizable mixture of one or more emulsion polymerizable monomers as described above, the neutralized form of 2-acrylamido-2-methylpropanesulfonic acid, surfactant, chelating agent and initiator. The neutralized form of 2-acrylamido-2-methylpropanesulfonic acid must be added in the seed step along with the comonomers at a pH greater than 4.5, preferably about 6 to 9 to be effective.
In a preferred embodiment, the anionic surfactant, chelating agent and neutralized form of 2-acrylamido-2-methylpropanesulfonic acid and one or more emulsion polymerizable monomers, are first added to the reactor, heated to about 150 degrees F and then an aqueous mixture of free radical initiator is added. The aqueous reactants are allowed to react and then the temperature is increased to about 170 degrees F.
Subsequently, aqueous emulsion polymerizable mixtures including at least one polymerizable monomer, about 0.5 to 2.0 wt. chain transfer agent and about 0 to 5 wt. surfactant are charged to the reaction zone of the reactor over a plurality of stages. In a preferred embodiment, the aqueous polymerizable mixtures are charged to the reactor in a batch at a rate faster than the polymerization rate over about six separate stages such that after each charge the mixture is allowed to react within the reactor. The additional stages include an aqueous polymerizable mixture of styrene, butadiene and chain transfer agent and optionally surfactant to stabilize growing particles. The emulsion polymerizable mixture is then allowed to react in the reactor to high conversion, preferably from 97% to nearly quantitative yield.
The invention will be further clarified by a consideration of the following examples, which are intended to be purely exemplary of the invention. As used in the Examples, Iam=N-Isopropylacrylamide; tBAm=N-t-butylacrylamide; Mam=methacrylamide; Peg-600DMA=A dimethylacrylate crosslinker with ""13 ethylene oxide units; TEGDMA=A dimethylacrylate crosslinker with ""3 ethylene oxide units; DAAm=diacetoneacrylamide; HMPA=hydroxypropylacrylate; MA=methylacrylate; EA=ethylacrylate; MMA=methylmethacrylate; ACN=acrylonitrile; NaSS=the sodium salt of styrene sulfonic acid; Na=sodium salt; NH4 =ammonium salt; NaAMPS=the sodium salt of 2-acrylamido-2-methylpropanesulfonic acid; Bd=1, 3-butadiene, and SBA=styrene, butadiene, acrylonitrile.